Allow to callback for MQTT subscription status

[jbouwh]

Proposed change

Integrations that want to be able to poll the state via MQTT when the integration starts, like Tasmota, need to be sure an MQTT subscription has been processed, before it can publish a ready state.

This PR allows allows to set a callback function when calling async_subscribe that is called when the subscription is done at the broker.

A second helper async_on_subscribe_done will allow to monitor subscriptions via a callback function. This helper will keep listening till the return callback handler is called.

Type of change

• Dependency upgrade
• Bugfix (non-breaking change which fixes an issue)
• New integration (thank you!)
• New feature (which adds functionality to an existing integration)
• Deprecation (breaking change to happen in the future)
• Breaking change (fix/feature causing existing functionality to break)
• Code quality improvements to existing code or addition of tests

Additional information

• This PR fixes or closes issue: fixes #
• This PR is related to issue:
• Link to documentation pull request:
• Link to developer documentation pull request: Blog post for new MQTT subscribe status callback option developers.home-assistant#2807
• Link to frontend pull request:

Checklist

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[home-assistant]

Hey there @emontnemery, @bdraco, mind taking a look at this pull request as it has been labeled with an integration (mqtt) you are listed as a code owner for? Thanks!

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[emontnemery]

Setting to draft while discussing the solution on Discord